printk: Introduce FORCE_CON flag

Introduce FORCE_CON flag to printk. The new flag will make it possible to
create a context where printk messages will never be suppressed.

This mechanism will be used in the next patch to create a force_con
context on sysrq handling, removing an existing workaround on the
loglevel global variable. The workaround existed to make sure that sysrq
header messages were sent to all consoles, but this doesn't work with
deferred messages because the loglevel might be restored to its original
value before a console flushes the messages.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20241105-printk-loud-con-v2-1-bd3ecdf7b0e4@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

include/linux/printk.h

@@ -166,6 +166,9 @@ __printf(1, 2) __cold int _printk_deferred(const char *fmt, ...);
 extern void __printk_deferred_enter(void);
 extern void __printk_deferred_exit(void);
 
+extern void printk_force_console_enter(void);
+extern void printk_force_console_exit(void);
+
 /*
  * The printk_deferred_enter/exit macros are available only as a hack for
  * some code paths that need to defer all printk console printing. Interrupts

kernel/printk/internal.h

@@ -53,6 +53,8 @@ int devkmsg_sysctl_set_loglvl(const struct ctl_table *table, int write,
 
 /* Flags for a single printk record. */
 enum printk_info_flags {
+	/* always show on console, ignore console_loglevel */
+	LOG_FORCE_CON	= 1,
 	LOG_NEWLINE	= 2,	/* text ended with a newline */
 	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
 };
@@ -90,6 +92,7 @@ bool printk_percpu_data_ready(void);
 
 void defer_console_output(void);
 bool is_printk_legacy_deferred(void);
+bool is_printk_force_console(void);
 
 u16 printk_parse_prefix(const char *text, int *level,
 			enum printk_info_flags *flags);

kernel/printk/printk.c

@@ -1319,11 +1319,11 @@ static void boot_delay_msec(int level)
 {
 	unsigned long long k;
 	unsigned long timeout;
+	bool suppress = !is_printk_force_console() &&
+			suppress_message_printing(level);
 
-	if ((boot_delay == 0 || system_state >= SYSTEM_RUNNING)
-		|| suppress_message_printing(level)) {
+	if ((boot_delay == 0 || system_state >= SYSTEM_RUNNING) || suppress)
 		return;
-	}
 
 	k = (unsigned long long)loops_per_msec * boot_delay;
 
@@ -2273,6 +2273,9 @@ int vprintk_store(int facility, int level,
 	if (dev_info)
 		flags |= LOG_NEWLINE;
 
+	if (is_printk_force_console())
+		flags |= LOG_FORCE_CON;
+
 	if (flags & LOG_CONT) {
 		prb_rec_init_wr(&r, reserve_size);
 		if (prb_reserve_in_last(&e, prb, &r, caller_id, PRINTKRB_RECORD_MAX)) {
@@ -2280,6 +2283,9 @@ int vprintk_store(int facility, int level,
 						 facility, &flags, fmt, args);
 			r.info->text_len += text_len;
 
+			if (flags & LOG_FORCE_CON)
+				r.info->flags |= LOG_FORCE_CON;
+
 			if (flags & LOG_NEWLINE) {
 				r.info->flags |= LOG_NEWLINE;
 				prb_final_commit(&e);
@@ -2947,6 +2953,7 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 	struct printk_info info;
 	struct printk_record r;
 	size_t len = 0;
+	bool force_con;
 
 	/*
 	 * Formatting extended messages requires a separate buffer, so use the
@@ -2965,9 +2972,13 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 
 	pmsg->seq = r.info->seq;
 	pmsg->dropped = r.info->seq - seq;
+	force_con = r.info->flags & LOG_FORCE_CON;
 
-	/* Skip record that has level above the console loglevel. */
-	if (may_suppress && suppress_message_printing(r.info->level))
+	/*
+	 * Skip records that are not forced to be printed on consoles and that
+	 * has level above the console loglevel.
+	 */
+	if (!force_con && may_suppress && suppress_message_printing(r.info->level))
 		goto out;
 
 	if (is_extended) {

kernel/printk/printk_safe.c

@@ -12,6 +12,24 @@
 
 #include "internal.h"
 
+/* Context where printk messages are never suppressed */
+static atomic_t force_con;
+
+void printk_force_console_enter(void)
+{
+	atomic_inc(&force_con);
+}
+
+void printk_force_console_exit(void)
+{
+	atomic_dec(&force_con);
+}
+
+bool is_printk_force_console(void)
+{
+	return atomic_read(&force_con);
+}
+
 static DEFINE_PER_CPU(int, printk_context);
 
 /* Can be preempted by NMI. */